The present invention relates to a phase change optical recording medium for recording and reproducing information by irradiating the medium with a light beam.
A phase change optical recording medium, in which recording and reproduction are carried out by irradiating the medium with a light beam, has advantages of large capacity, high-speed access and medium portability. As the recording density of the medium increases, the field of application thereof is expected to be broadened. The operation of the phase change optical recording medium is as follows. In recording, an optical recording layer in a crystalline state is irradiated with a light beam of a recording power level to heat the recording site up to a temperature above the melting point thereof, and then the irradiated site is rapidly cooled in a short period of time less than the crystallization time, thereby forming an amorphous recording mark. In this case, overwriting can be performed by light intensity modulation. The recorded information is readout by utilizing the difference in the reflectance between the crystalline region and the amorphous recording mark.
In order to obtain a satisfactory overwrite performance (including .GAMMA. characteristics and overwrite repeatability), the phase change optical recording medium should preferably have a stacked structure which allows rapid heating and rapid cooling. FIG. 1 shows a typical structure of a conventional phase change optical recording medium which is now in practical use. As shown in FIG. 1, on a substrate 1, there are formed a first interference layer (lower protective layer) 2 made from a relatively thick dielectric having a thickness of 100 to 200 nm, a thin optical recording layer 3 having a thickness of 10 to 30 nm, a second interference layer (upper protective layer) 4 made from a relatively thin dielectric having a thickness of 10 to 40 nm and a reflective layer 5 made from a relatively thick and highly thermal conductive metal having a thickness of 50 to 100 nm.
As a technique which further enhances the recording density of the phase change optical recording medium, pulse width modulation recording (mark-edge detection) and land-groove (L-G) recording are known. The pulse width modulation recording makes it possible to reduce bit pitch by recording data in the edges of recording marks. According to this method, the recording density can be about 1.5 times that of conventional mark position recording. In the land-groove recording, the depth of groove is set to about 1/6 of the laser wavelength so as to reduce cross talk, which allows to record data on both land and groove. According to the L-G recording, the recording density can be about twice that of the conventional method in which data is recorded only in either land or groove.
In the L-G recording, it is required to suppress cross erase, i.e., a phenomenon that the recording mark edges in adjacent tracks are erased. Since the optical recording medium of FIG. 1 has a stacked structure capable of rapid heating and cooling, which effectively suppresses cross erase, there is little trouble in so far as the L-G recording is concerned. Meanwhile, in order to attain the pulse width modulation recording, it is required to minimize fluctuation in mark edge position. However, since the structure of FIG. 1 is likely to cause fluctuation in mark edge position, the pulse width recording is hard to realize. The reason is explained as follows. As to the recording layer alone, the reflectance of an amorphous region is smaller than that of a crystalline region. Besides, in the structure of FIG. 1, light passed through the recording layer is totally reflected by the uppermost reflective layer 5 and returned to the recording layer 3. Taking these conditions into consideration, the effective absorbance (A*) of the recording layer, which is observed from the incident side of light beam, is greater in the amorphous region (Aa*) than in the crystalline region (Ac*). It is problematic to carry out overwriting under the condition of Aa*&gt;Ac*. That is, since the crystalline region is slowly heated up due to the smaller absorption and also requires a latent heat of melting, the region is hard to melt relative to the amorphous region. Therefore, the size of recording mark to be newly formed differs depending on whether the overwritten site is crystalline or amorphous, which means the fluctuation in mark edge position becomes greater.
Accordingly, in order to attain the pulse width modulation recording by suppressing the fluctuation in mark edge position upon overwriting, it has been found desirable that the condition of Aa*.ltoreq.Ac* be met. Under the circumstances, optical recording media having an improved stacked structure so as to satisfy the above condition have been proposed as follows.
(1) A stacked structure similar to that of FIG. 1 except that the uppermost reflective layer is replaced with a semitransparent layer: See, for example, ISOM/ODS-joint international conference proceeding, pp.71 (Th.3.5). This stacked structure meets the condition of Aa*.ltoreq.Ac* by allowing a part of light to transmit through the semitransparent layer.
(2) A stacked structure in which a semi-transparent layer is inserted between the substrate and the first interference layer in addition to that of FIG. 1: See, for example, U.S. Pat. No. 5,431,978. This stacked structure realizes the condition of Aa*.ltoreq.Ac* by utilizing interference of light.
In the above optical recording media, however, the semitransparent layer reduces heat radiation, which makes thermal response slow. As a result, overwrite repeatability is deteriorated due to increase in thermal load on the recording layer as well as the land-groove recording characteristics is deteriorated due to increase in cross erase.
As stated above, it has been difficult for conventional optical recording media to meet excellent overwrite repeatability and land-groove recording as well as excellent pulse width modulation recording.